* Fix sampling and Kafka reporter conflicting.

* Remove legacy codes.
Co-authored-by: Nkezhenxu94 <kezhenxu94@apache.org>

Co-authored-by: NDaming <zteny@foxmail.com>

Support @KafkaPollAndInvoke annotation to replace @Trace when just using the Kafka scenario agent plugin (#5304)

* disable kafka plugin

* enable docker file of kafka for e2e test

use the  method httpMethod.setRequestHeader to set the new skywalking headers,so that it can override the exits headers if there is any
Co-authored-by: 吴晟 Wu Sheng <wu.sheng@foxmail.com>

* [maven-release-plugin] prepare release v8.1.0

* [maven-release-plugin] prepare for next development iteration

Another thread could get the list which is still in the initialization stage, due to first element added. So we need a local variable to avoid this. Also, notice this happens only in concurrency class loader env.